Fibers cross-linked in substantially individualized form and various methods for making such fibers have been described in the art. The term “individualized, cross-linked fibers”, refers to cellulosic fibers that have primarily intrafiber chemical crosslink bonds. That is, the crosslink bonds are primarily between cellulose molecules of a single fiber, rather than between separate fibers. Individualized, cross-linked fibers are generally useful in absorbent product applications. The fibers themselves and absorbent structures containing them fibers generally exhibit an improvement in at least one significant absorbency property relative to conventional, uncross-linked fibers. Often, the improvement in absorbency is reported in terms of absorbent capacity. Additionally, absorbent structures made from individualized cross-linked fibers generally exhibit increased wet resilience and increased dry resilience relative to absorbent structures made from uncross-linked fibers. The term “resilience” refers to the ability of pads made from cellulosic fibers to return toward an expanded original state upon release of a compressional force. Dry resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a substantially dry condition. Wet resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a moistened condition.
With cross-linked fibers it is important that the cross-linking agent penetrates and distributes thoroughly within the interior of the individual fiber structure prior to cross-linking or during the cross-linking process. Insufficient penetration and distribution within the fibers will result in reduced intrafiber cross-links and compromise performance properties of the cross-linked fibers and absorbent structures formed from them.
In general, three categories of processes have been reported for making individualized, cross-linked fibers. These processes, described below, are herein referred to as dry cross-linking processes, aqueous solution cross-linking processes, and substantially non-aqueous solution cross-linking processes.
Processes for making individualized, cross-linked fibers with dry cross-linking technology are described in U.S. Pat. No. 3,224,926, L. J. Bernardin. Individualized, cross-linked fibers are produced by spraying cellulose drylap with cross-linking agent, defiberizing the fibers by mechanical action, and drying the fibers at elevated temperature to effect cross-linking while the fibers are in a substantially individual state. The fibers are inherently cross-linked in an unswollen, collapsed state as a result of being dehydrated prior to cross-linking. Processes as exemplified in U.S. Pat. No. 3,224,926, wherein cross-linking is caused to occur while the fibers are in an unswollen, collapsed state, are referred to as processes for making “dry cross-linked” fibers. Dry cross-linked fibers are generally highly stiffened by cross-link bonds, and absorbent structures made therefrom exhibit relatively high wet and dry resilience. Dry cross-linked fibers are further characterized by low fluid retention values.
Processes for producing aqueous solution cross-linked fibers are disclosed, for example, in U.S. Pat. No. 3,241,553, F. H. Steiger. Individualized, cross-linked fibers are produced by cross-linking the fibers in an aqueous solution containing a cross-linking agent and a catalyst. Fibers produced in this manner are hereinafter referred to as “aqueous solution cross-linked” fibers. Due to the swelling effect of water on cellulosic fibers, aqueous solution cross-linked fibers are cross-linked while in an uncollapsed, swollen state. Relative to dry cross-linked fibers, aqueous solution cross-linked fibers as disclosed in U.S. Pat. No. 3,241,553 have greater flexibility and less stiffness, and are characterized by higher fluid retention value (FRV). Absorbent structures made from aqueous solution cross-linked fibers exhibit lower wet and dry resilience than structures made from dry cross-linked fibers.
Cross-linked fibers as described above are believed to be useful for lower density absorbent product applications such as diapers and also higher density absorbent product applications such as catamenials. However, such fibers have not provided sufficient absorbency benefits, in view of their detriments and costs, over conventional fibers to result in significant commercial success. Commercial appeal of fibers cross-linked with cross-linking agents referred to in the literature as formaldehyde and formaldehyde addition products has also suffered due to safety concerns.
The use of specific polycarboxylic acids to crosslink cellulosic fibers is known in, for example, U.S. Pat. Nos. 5,137,537, 5,183,707, and U.S. Pat. No. 5,190,563, all to Herron et al. Herron discloses absorbent structures containing individualized cellulosic fibers cross-linked with a C2-C9 polycarboxylic acid. The ester crosslink bonds formed by the polycarboxylic acid cross-linking agents are different from the crosslink bonds that result from the mono- and di-aldehyde cross-linking agents, which form acetal cross-linked bonds. Unlike formaldehyde and formaldehyde addition products, the C2-C9 polycarboxylic acid cross-linking agents are non toxic and safe for use on human skin. One preferred polycarboxylic cross-linking agent i.e., citric acid, is available in large quantities at relatively low prices making it commercially competitive with formaldehyde and formaldehyde addition products. Unfortunately, citric acid can require a long curing time and a large amount of catalyst to promote the cross-linking reaction which increases cost. Additionally, at elevated temperatures citric acid becomes unstable, discolors (i.e., yellows) the white cellulosic fibers and unpleasant odors can be produced.
The use of polymeric polycarboxylic acids to crosslink cellulosic fibers is also known in, for example, EP Patent No. 0765416B1 and U.S. Pat. No. 5,549,791. These references disclose individualized cellulosic fibers cross-linked with a polymeric polyacrylic acid cross-linking agent having a molecular weight of from 500 to 40,000 where the cross-linking agent is polyacrylic acid polymer, a copolymer of acrylic acid and maleic acid, and copolymers of polyacrylic acid and polymeric monoalkyl phosphinates and polymeric monoalkyl phosphonates. The polymeric polyacrylic cross-linking agents described are particularly suitable for forming ester crosslink bonds with cellulosic fibers. Importantly, the ester-cross-linked fibers tend to be brighter than those cross-linked with alphahydroxy acids such as citric acid. Furthermore, the polymeric polyacrylic cross-linking agents are stable at higher temperatures, thus promoting more efficient cross-linking.
The use of polymeric polycarboxylic acids cross-linking agents to prepare intrafiber cross-linked cellulose fibers and absorbent structures made therefrom appear to overcome many of the disadvantages associated with formaldehyde and/or formaldehyde addition products and C2-C9 polycarboxylic acids cross-linking agents. However, the cost associated with producing fibers cross-linked with the polymeric polycarboxylic cross-linking agents may be too high to result in significant commercial success. Therefore, there is still a need to find cellulosic fiber cross-linking agents which are safe for use on the human skin, provide absorbent structures with high fluid absorption properties and also are commercially feasible.
It is an object of this invention to provide individualized fibers cross-linked with a phosphinate-containing telomer of acrylic acid cross-linking agent and absorbent structures made from such fibers wherein the absorbent structures made from the cross-linked fibers have higher levels of absorbent capacity relative to absorbent structures made from prior known polymeric polycarboxylic acid cross-linked fibers and exhibit higher resilience relative to absorbent structures made from prior known polymeric polycarboxylic acid cross-linked fibers.